How To Customize a Tumblr Theme?

Okay So Heres My Question, Well. first.
i do know some CSS, and HTML. But I’ve been
searching for a while on how to
figure out the Tumblr Style Sheet, And still, i have no idea.
Mainly, Positioning The Posts, (i want my posts to be in a div, not going way down the page)
Editing The posts (Audio, Pictures, Text, ...)
And Also Positioning the divs for my about me. and adding another one for my links, or whatever i want to put.
So please help me understand this.